1. Field of the Invention
The present invention relates to a structure for protecting an electronic circuit connected to a telephone connection line against fast overvoltages, for example due to lightning.
2. Discussion of the Related Art
FIG. 1 schematically shows an electronic circuit 1 connected to rails 3 and 5 of a telephone connection 7. Circuit 1 is capable of transmitting and/or of receiving signals, respectively VTIP and VRING, over rails 3 and 5. Signals VTIP and VRING for example are speech signals, ringing signals, etc. Circuit 1 is further connected to a power supply terminal 9 of high voltage VH and to a power supply terminal 11 of low voltage VL. Circuit 1 for example is a SLIC-type circuit (“Subscriber Line Interface Circuit”), capable of behaving as an interface between an analog telephone connection line and digital telephone network equipment.
Short and abrupt overvoltages, for example due to lightning, may occur on rails 3 and/or 5. Such overvoltages are capable of damaging components of circuit 1. It is thus generally provided to connect to telephone line 7, between rails 3 and 5, a protection structure 13, capable of rapidly draining off significant currents that may appear when an overvoltage occurs on rail 3 and/or on rail 5.
In an example, structure 13 comprises thyristors 15, 17, 19, and 21, respectively forward-connected between the ground and rail 3, between rail 3 and the ground, between the ground and rail 5, and between rail 5 and the ground. Structure 13 further comprises zener diodes 23, 25, 27, and 29, respectively forward-connected between a cathode gate of thyristor 15 and the ground, between the ground and an anode gate of thyristor 17, between a cathode gate of thyristor 19 and the ground, and between the ground and an anode gate of thyristor 21.
It should be noted that term “ground” here designates a reference potential common to all the device elements, for example, a potential close to 0 V. In practice, structure 13 may be grounded via a ground terminal of circuit 1, or via a ground rail (not shown) comprised in connection 7. In the following description, “positive potential” and “negative potential” will be used to designate potentials respectively greater than the ground potential and smaller than the ground potential, and each time digital potential values will be given as an example, these values will be considered to refer to a ground potential equal to 0 V.
In case of a positive overvoltage on rail 3, thyristor 17, which is forward biased, is capable of being turned on. If the overvoltage exceeds a given threshold, zener diode 25 turns on by avalanche effect. A current then flows between rail 3 and the ground, through the PN junction, between the anode and the anode gate of thyristor 17, and through zener diode 25. Thyristor 17 is thus turned on and the overvoltage is removed towards the ground.
In case of a negative overvoltage on rail 3, thyristor 15 is capable of being turned on. If the overvoltage exceeds a given threshold, zener diode 23 turns on by avalanche effect for the negative overvoltage. A negative current then flows between rail 3 and the ground, through the PN junction between the cathode gate and the cathode of thyristor 15. Thyristor 15 is thus turned on and the overvoltage is removed towards the ground.
In case of a positive or negative overvoltage on rail 5, a similar removal scheme applies through thyristors 21 or 19 and zener diodes 29 or 27. Thus, structure 13 enables to remove any overvoltage that may occur on rails 3 and 5.
The turn-on threshold for a positive overvoltage is thus equal to the avalanche voltage of a zener diode (25 or 29) plus the forward voltage drop of a PN junction (on the order of 0.6 V). The turn-on threshold for a negative overvoltage is equal to the opposite of the avalanche voltage of a zener diode (23 or 27) minus the forward voltage drop of a PN junction.
A disadvantage of this type of structure is that the avalanche voltages of the zener diodes should be adapted to the maximum and minimum values that may be taken by signals VTIP and VRING in a normal operation of the device. This actually results in selecting avalanche voltages much greater than the normal excursions of signals VTIP and VRING with respect to the ground, to take into account the component dispersion.
FIG. 2 is an electric diagram corresponding to the diagram of FIG. 1, where protection structure 13 with fixed turn-on thresholds has been replaced with a protection structure 31 having its turn-on thresholds associated with the power supply voltages of circuit 1. Structure 31 is connected not only to rails 3 and 5, but also to power supply terminals 9 and 11 of circuit 1 to be protected. It comprises thyristors 15, 17, 19, and 21, connected as in FIG. 1. Structure 31 further comprises an NPN transistor 33, a PNP transistor 35, an NPN transistor 37, and a PNP transistor 39. The emitters of transistors 33, 35, 37, and 39 are respectively connected to the cathode gate of thyristor 15, to the anode gate of thyristor 17, to the cathode gate of thyristor 19, and to the anode gate of thyristor 21. The collectors of these transistors are all grounded. The bases of transistors 33 and 37 are connected to low power supply terminal 11 (VL), and the bases of transistors 35 and 39 are connected to high power supply terminal 9 (VH).
In case of a positive overvoltage on rail 3, thyristor 17, which is forward biased, is capable of being turned on. The overvoltage is transferred by the PN junction between the anode and the anode gate of thyristor 17 onto the emitter of transistor 35. If the overvoltage exceeds a given threshold, the voltage of the emitter of transistor 35 exceeds the base voltage of this transistor (that is, VH), which turns on. A current then flows between rail 3 and the ground, from the anode to the anode gate of thyristor 17, and through transistor 35. Thyristor 17 is thus turned on and the overvoltage is removed towards the ground.
The other overvoltage polarities are similarly removed by one of the other thyristors, noting that negative overvoltages are referenced to a threshold associated with low power supply voltage VL.
The turn-on threshold for a positive overvoltage is thus equal to high power supply voltage VH plus twice the forward voltage drop of a PN junction (on the order of 1.2 V). The turn-on threshold for a negative overvoltage is equal to low power supply voltage VL minus twice the forward voltage drop of a PN junction.
This type of structure is used when the power supply voltages VH and VL of the circuit to be protected, present on terminals of the circuit, correspond to the maximum and minimum values that may be taken, in normal operation, by signals VTIP and VRING. This type of structure is also used when the circuit to be protected comprises reference terminals between which a reference voltage corresponding to the voltage level of the signals present on the line is established. However, structure 31 has the disadvantage of disturbing the voltages of the reference terminals to which it is connected.
In certain cases, the circuit to be protected receives on its access terminals a power supply voltage of much lower level than the voltage level of signals VTIP and VRING. The circuit to be protected then comprises converters for providing voltage levels adapted to the telephone line, and these voltage levels are not accessible from access terminals of the circuit.